There are many examples of a process that emits a multi-component mixture of gases. One such process, combustion in an internal combustion engine, is a chemical reaction that has substantial practical importance and has consequently been studied in detail. The combustion that occurs within the cylinder of an engine is a chemical reaction in which a hydrocarbon fuel is oxidized. A multi-component mixture of gases is emitted by this reaction in the form of the engine exhaust. The products of this reaction, however, include not only the exhaust gases themselves but also the work done in moving the piston in the cylinder. Various components of an engine, including the emissions control devices, thus each acts as a device to which a product of the chemical reaction is transmitted. Although combustion of a hydrocarbon fuel is a relatively simple type of chemical reaction, the manner in which the force of the gases produced by that reaction expand against a piston to power an engine and all of its associated components is more complicated and requires control. In the case of a modern automotive engine, control is accomplished by digital processing computers in an engine control unit (“ECU”) that rely on inputs from a variety of sensors and actuators.
The underlying objective of the ECU is to provide performance that optimizes fuel efficiency, drivability and reduction of harmful emissions. Signals concerning the state or condition of various operating characteristics of the engine are fed to the ECU. Typical engine operating characteristics as to which signals are inputted to the ECU are throttle position, intake manifold pressure, intake airflow, crank position, engine torque and air-to-fuel ratio (referred to as “lambda”) value. Engine operating characteristics that may be adjusted for control in view of such inputs include fuel injection timing, spark advance, air-to-fuel ratio, exhaust gas recycle (“EGR”) and idle air control motor. Although an engine is essentially a chemical plant oxidizing fuel with air into water, carbon dioxide and other chemical species, the only sensor currently capable of providing any information about the chemical status of the combustion process is the lambda sensor, which is limited to inferring a value for the air-to-fuel ratio of the engine based on measurements made in the stream of exhaust gases.
Much work has been done to develop relationships between the signals inputted to an ECU, and the operating characteristics that are thereby controlled, to optimize engine performance. This work is based on theoretical models of the combustion process, engine dynamics and other power train components. See, for example, Arsie, Pianese and Rizzo, Models for the Prediction of Performance and Emissions in a Spark Ingnition Engine—A Sequentially Structured Approach, SAE Paper 980779, 1998. Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw Hill (1988). Pulkrabek, W. W., Engineering Fundamentals of the Internal Combustion Engine, Prentice Hall (1997). These models attempt to predict both the engine operating characteristics and the chemical components of the exhaust stream, but they tend to be quite complex and yield only approximate results. For this reason, an empirical system of control has been adopted that uses engine mapping.
Calibrating engine operation, either with or without an attached transmission, creates a map, which records empirically-observed relationships between one or more operating characteristics as to which information is gathered and inputted to the ECU, and one or more operating characteristics that are adjusted in view of the inputted information. For example, FIG. 4 shows a map relating speed to load to measurements concerning the presence of nitrogen oxides (NOx) in the engine exhaust. It may be seen how moving along the surface defined by the map can move an engine from one operating state to another. In doing so, the value of at least one of the parameters may be held constant, if desired.
In current engine design technology, a map that includes engine exhaust gases is commonly used to estimate the emission of pollutants over a wide range of conditions. The assumption is made that the map obtained during original calibration is either stable or undergoes predictable change, in which case adaptive algorithms are used to estimate the model change. By using the relationship between input signals, controlled operating characteristics, and the mapped emission levels, engines have been operated on the assumption that the predicted relationship between mechanical performance and exhaust content is accurate, and thus that emission control of a desired nature results from control of the same operating characteristics that control mechanical performance of the engine.
It has been found, however, that control systems using engine mapping provide control only as good as the input signals and the validity of the map. As engines wear, sensors lose calibration, fuel compositions change, and the assumption made that the fundamental combustion process and the content of the exhaust stream remain stable becomes invalid. Precision analytical equipment that could provide a complete analysis of the engine exhaust gasses, and that could provide information that remains accurate during service of the engine in real time, is not practical for use for such purpose except during the original calibration of a map in a laboratory.
These deficiencies are particularly acute with respect to a map that incorporates lambda, the air-to-fuel ratio, as an input. There are two common types of lambda sensors: step-change lambda sensors and wide-range lambda sensors. The step-change lambda sensor is based on a zirconia concentration cell and operates between λ=0.95 and λ=1.05. This sensor is used for air/fuel ratio control in engines that operate around stoichiometry (λ=1). It is desirable to operate at λ=1 because the catalytic converter operates best with a stoichiometric mixture. The wide-range lambda sensor operates over a much wider range of lambda, and it enables the closed-loop control of lean-burn engines. Running lean (oxygen-rich) is important to ensure that all of the fuel is combusted.
A lambda sensor such as described above generates a single signal that indicates whether the exhaust gas is rich or lean (and in the case of the wide-range sensor, indicates to what extent rich or lean). This single signal is derived from a composite of all gases in the exhaust stream, reflecting the ratio of oxidizing to reducing gases therein. This type of lambda sensor is not capable of providing detailed information about the gas composition of an exhaust stream, and a lambda value derived from this sensor is not an indication of a unique gas composition. Different combinations of gases can produce the same lambda value. This type of lambda sensor is sometimes also referred to as an oxygen sensor because oxygen diffuses readily through the zirconia cell, but this sensor does not furnish any information about the individual concentration of oxygen as an individual component within a stream of exhaust gas. Even when a metal oxide film that has a high diffusion coefficient for oxygen is used as a lambda sensor, the resulting lambda value does not furnish useful information about the individual concentration of oxygen as an individual component within a stream of exhaust gas because the cross-sensitivity of the metal oxide film requires that assumptions be made about the extent to which other components may be present in the exhaust stream, or about the conditions under which combustion has occurred.
A lambda sensor may also be used in the monitoring and control of a nitrogen oxide (NOx) absorber in a lean-burn engine. As described, for example, in U.S. Pat. No. 6,216,448, an oxygen deficiency in the exhaust gas downstream of a two part catalytic converter, which contains an upstream conventional oxygen storage section and a downstream NOx storage section, only occurs when the oxygen-storage locations as well as the nitrous oxide storage locations in the NOx storage catalytic converter are empty. These locations are emptied by the passage through them of a rich mixture. The time difference in the response of upstream and downstream sensors to the rich exhaust mixture is used as a measure of the NOx, storage capacity. This is not, however, a measurement of NOx content in the exhaust gas stream.
It would therefore be desirable to have methods and apparatus for controlling a process (such as a chemical reaction) that emits a multi-component mixture of gases, or a device (such as an internal combustion engine) to which is transmitted a product of a chemical reaction that emits a multi-component mixture of gases. These methods and apparatus receive as an input, and optionally utilize in a map, information about the individual concentration within the emitted gas stream of one or more individual component gases, or subgroups of gases, therein.